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Gravity Probe B (GP-B) is a satellite-based mission which launched on 20 April 2004. The...

Gravity Probe B (GP-B) is a satellite-based mission which launched on 20 April 2004. The spaceflight phase lasted until 2005, and data analysis is expected to continue through 2010. Its aim is to measure spacetime curvature near Earth, and thereby the stress-energy tensor (which is related to the distribution and the motion of matter in space) in and near Earth. This will provide a test of general relativity and related models. Initial results confirmed the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging effect was similar in magnitude to the current noise level (the noise being dominated by initially unmodeled effects). Work is continuing potentially to as late as March 2010, to model and account for these sources of unintended signal, thus permitting extraction of the frame-dragging signal if it exists at the expected level. By August 2008 the uncertainty in the frame-dragging signal had been reduced to 15%, and the December 2008 NASA report indicated that the geodetic effect was confirmed to better than 0.5%. Gravity Probe B was a relativity gyroscope experiment funded by NASA. Efforts were led by Stanford University physics department with